1. Field of the Invention
The present invention relates to a film formation apparatus for a semiconductor process for forming a silicon film on target objects, such as semiconductor wafers, and a method for using the apparatus. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target object, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target object.
2. Description of the Related Art
As a semiconductor device manufacturing apparatus for performing a heat process on the surface of a target object, such as a semiconductor wafer (which may be simply referred to as a wafer), there is a vertical heat processing apparatus of the hot wall type, which is a so-called batch furnace. A vertical heat processing apparatus includes a vertical reaction tube or reaction container made of, e.g., quartz with a heater disposed around it. A number of wafers are held on a holder or wafer boat as in shelves and are loaded into the reaction tube. A process gas is supplied into the reaction tube while the reaction tube is heated by the heater, so that a heat process is performed on the wafers all together.
FIG. 12 is a sectional side view showing a vertical heat processing apparatus (silicon film formation apparatus) conventionally used. This apparatus includes a vertical reaction tube 1 disposed inside a furnace 17a having a heater 17 extending on the inner surface. A wafer boat 21 is placed on a lid 2 and is moved up and down by the lid 2 between a position inside the reaction tube 1 and a position below the reaction tube 1. Gas supply lines are inserted in the flange of the reaction tube 1 and an exhaust port 15 is formed at the top of the reaction tube 1 such that gas flows inside the reaction tube 1 from the bottom to the top. In FIG. 12, only a gas supply line 12 for mono-silane gas and a gas supply line 14 for hydrogen fluoride gas are shown as part of various gas supply lines, for the sake of convenience. The reaction tube 1 and wafer boat 21 are made of quartz. One of the reasons for this is that it is necessary to form an Si (silicon) atmosphere inside the reaction tube 1 when silicon epitaxial growth is performed on silicon wafers.
When silicon epitaxial growth is performed on silicon wafers W, silicon needs to be exposed on the surface of the wafers W. However, when the wafers W are transferred under atmospheric air, natural oxide films (silicon oxide films), which hinder the epitaxial growth, are produced on the surface. Accordingly, the natural oxide films on the surface of the wafers W are etched and thereby removed by supplying a mixture gas of hydrogen fluoride (HF) gas and ammonia (NH3) gas. Then, the silicon epitaxial growth is performed on the wafers W from which the natural oxide films have been removed.
Where hydrogen fluoride (HF) gas is solely used for etching natural oxide films, reactions are caused as shown in the following formula (1). On the other hand, where hydrogen fluoride (HF) gas and ammonia (NH3) gas are used for etching natural oxide films, reactions are caused as shown in the following formulas (2) and (3).SiO2+4HF→SiF4+2H2O  (1)SiO2+4HF+4NH3→SiF4+2H2O+4NH3  (2)SiF4+2HF+2NH3→SiF4+(NH4)2SiF6  (3)
The formula (1) needs reaction energy of 1.04 eV, while the formulas (2) and (3) need reaction energy of 0.49 eV and 0 eV, respectively. Further, (NH4)2SiF6 (ammonium silicofluoride) generated in the formula (3) sublimes at about 200° C. and is thereby easily removed along with exhaust gas. In other words, the hydrogen fluoride gas serves as a gas for etching the natural oxide films, while the ammonia gas serves as a gas for lowering the necessary reaction energy for this etching and promoting vaporization of products generated by this etching (i.e., increasing the etching rate and improving the selectivity of removal for natural oxide films). Accordingly, in the explanation hereinafter, the hydrogen fluoride gas is referred to as an etching gas, while the ammonia gas is referred to as a vaporization promoting gas, as needed.
However, as described later, the present inventors have found that conventional methods for using a film formation apparatus for a semiconductor process of this kind have room for improvement in terms of problems concerning particle generation.